Caracterização de sensores termo-resistivos.

Abstract

This thesis constitutes a contribution to the study of charaterization of thermoresistive sensors. The intrinsic thermal time constant of a thermoresistive sensor is defined in terms of the physical parameters and proprietes like mass (m), area (A), specific heat (c) and global heat transfer coefficient (U). These quantities are not easy to determine. In general the composition of the sensor is not homogeneous and involves invariably more than one material. The format is also not very regular shaped. There is thus no easy and straight forward method to determine m and A. Experimental procedures for the determination of mc, UA and I are outlined. Some of these procedures involve the determination of the step response of the sensor to incident radiation or equivalent Joule heating. The electrical heating of the sensor introduces error in the experimental measurement of the dynamic response. The determination of the time constant by subjecting it to incident radiation, also requires an electrical current to monitor the resistance measurements. This current also introduces an error in the measurement of the time constant. This error has been quantified by using a mathematical formulation of the sensor's operation. An experimental method is then proposed to eliminate the above error and besides determine the value of mc and UA. Preliminary sensitivity analyses for X, mc and UA, help in choosing the resistance measurement or heating currents. The use of recursive parametric identification technique is applied to determine the sensor parameters. The formulation of the sensor operation in an automatically balanced feedback bridge circuit is also presented, from which it is possible to calculate the response time of the overall instrument for measurement of solar radiation.